I appear to be having that same issues with FG... there's a box that appears around the ICE volume cloud when I use Mental Ray with FG enabled. For the environment I am just using a sphere with a simple constant material applied and some blue in the incandescence channel.

I did find that setting the Primary Rays to "Rasterizer" solves the problem but there's definitely a substantial decrease in speed. Hoping there's a faster solution for this.

Hmmm... I guess it may have something to do with the fact I was using textured geometry as the sky. I found that inside the sky sphere geo/Visibility/Rendering, disabling Secondary Rays or disabling Final Gathering/Sampled seems to have done the trick.

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